1. Field of the Invention
The present invention relates to a finder optical system of a single lens reflex camera.
2. Description of the Related Art
A finder optical system of a single lens reflex camera (hereinafter, a SLR camera), in which the diopter suitable for an operator can be adjusted by moving a diopter adjusting lens element being a part of an eyepiece optical system along the optical axis, is known in the art. For example, in U.S. Pat. No. 4,389,097, a SLR camera which employs a penta prism as an erecting optical system is provided with a finder optical system including a negative lens element, a positive lens element, a positive lens element and a negative lens element, the two positive lens elements of which are moved to vary a focal length of the eyepiece optical system whereby the diopter is adjusted.
Furthermore, in a SLR camera, in order to comply with cost and weight reduction requirements, a `penta-mirror,` i.e., an assembly of plurality of mirrors, through which light ray is reflected in the same way as a penta prism to erect an image is known in the art. For example, U.S. Pat. No. 5,136,427 and Japanese Unexamined Patent Publication No.9-146019 disclose an eyepiece optical system of a SLR camera having this type of penta-mirror.
When a penta-mirror is used in an erecting optical system, the equivalent air optical path becomes approximately 1.5 times (multiplication of a refractive index of the penta prism to be compared) longer than that of a penta prism under the same conditions wherein the penta-mirror and the penta prism have the same geometric distances of the reflection optical paths thereof. Consequently, the focal length of a finder optical system becomes long, which causes a magnification reduction of the finder optical system. In order to solve this problem, in U.S. Pat. No. 5,136,427, a lens element having a strongly powered concave surface is provided on the side of an operator's eye whereby (i) the principal points are positioned as close to the penta-mirror as possible, (ii) the focal length of the eyepiece optical system is made short, and (iii) magnification reduction of the finder optical system is prevented.
Also, in Japanese Unexamined Patent Publication No. 9-146019, a negative lens element having a strongly powered concave surface is provided on the side of the operator's eye, and a strongly powered positive lens element is provided on the penta-mirror side whereby (i) the principal points are positioned as close to the penta-mirror as possible, (ii) the focal length of the eyepiece optical system is made short, and (iii) a magnification reduction of the finder optical system is prevented. Furthermore, by varying the distance between the negative and positive lens elements, the diopter with respect to the focusing screen on which the photographing optical system of a SLR camera forms a real image can suitably be adjusted for an individual operator.
However, in Japanese Unexamined Patent Publication No. 9-146019, upon adjusting the diopter, since the strongly powered positive and negative lens elements are relatively moved, the apparent visual angle is largely varied as well. The adjusting of the diopter is usually performed while an operator is looking through the finder, and accordingly, unpleasant for the operator to observe the apparent visual angle being varied. In particular, when the diopter is being suitably set for the operator, if the apparent visual angle is narrowed, the feel of unpleasantness may be intensified.